Films such as optical thin films for optical filters, antireflection films of various displays, and films of various substrate with a thin film used in various semiconductors, optical discs, LCDs, color filters, transparent electrodes, etc. are formed by vacuum evaporation, ion assisted evaporation, ion plating, sputtering, ablation, etc.
In reference to a case where an antireflection film is formed on an optical lens or where an optical thin film such as an antireflection film or optical filter is formed on a flat substrate such as a glass sheet or resin sheet by vacuum evaporation, the conventional method for manufacturing substrate with a thin film is described below in reference to drawings.
FIG. 4 is a typical view showing how such substrate with a thin film are produced. At first, a film forming chamber is described. Inside a vacuum chamber 16, a film material 3 is placed with its film forming particle generating region 2 turned upward in the drawing. An electron beam is emitted from an electron gun 6 to reach the film forming particle generating region 2 due to the effect of a magnetic field not illustrated, to heat the region 2, and as a result, film forming particles 5 are generated. The film forming particle flux axis 9 (directional axis to express the direction in which the film forming particles are generated most intensively) of the film forming particles 5 generated like this is turned upward in the drawing. On or near the axial line of the film forming particle flux axis 9, a film formation monitor 8 is installed, and a film thickness measuring instrument 7 for optically measuring the thickness of the film formed on the film formation monitor 8 is installed further above the monitor 8. To allow the film forming particles to reach the film formation monitor 8, a hole 401 is formed at the center of a domed substrate holder 402. The domed substrate holder 402 can rotate on a horizontal plane above the film forming particle generation source, so that the under surfaces of the substrates on which a film to be formed 403 fixed in the domed substrate holder 402 may be exposed to the film forming particles, to form a film on each of them. Furthermore, to limit the range in which the substrates on which a film to be formed 403 are exposed to the film forming particles 5, a film forming range limiting member 4 is installed at a position between the film forming particle generating region 2 and the substrates on which a film to be formed 403. A shutter 11 or intercepting the film forming particles 5 as required is also similarly installed.
FIG. 5 is a drawing showing this state from the position of the film formation monitor 8 in the direction toward the film forming particle flux axis 9. The substrates on which a film to be formed 403 are arranged, for example as illustrated, to be held by the domed substrate holder by using the available area of the holder to the maximum extent, and the hole 401 is formed on or near the axial line of the film forming particle flux axis 9, to prevent the film forming particles 5 from being intercepted by the substrates, etc. The film formation monitor 8 is arranged to allow the film forming particles to reach the film formation monitor 8 through the hole.
To form a thin film on the under surface of each of the respective substrates on which a film to be formed 403, at first, the domed substrate holder 402 is kept rotating beforehand. Then, the electron beam emitted from the electron gun 6 continuously heats the film forming particle generating region 2 of the film material 3, to generate the film forming particles from there. In this case, at first, since the shutter 11 is closed, the film forming particles cannot reach the substrates on which a film to be formed. When the temperature of the film forming particle generating region 2 reaches a steady state, the film forming particle generation intensity also reaches a steady state. After this has been confirmed, the shutter 11 is opened. The film forming particles generated from the film forming particle generating region 2 are flown radially with the direction of the film forming particle flux axis 9 as the central direction, and reach the respective substrates on which a film to be formed 403. Furthermore, through the hole 401 formed on or near the axial line of the film forming particle flux axis 9, some film forming particles reach the film formation monitor 8.
The thickness of the film formed on each of the respective substrates on which a film to be formed 403 is indirectly measured by measuring the thickness of the film formed on the film formation monitor 8 by the film thickness measuring instrument 7. On the film formation monitor 8, a film is formed under conditions approximating those for forming the film on each of the respective substrates on which a film to be formed 403. So, the thickness of the thin film formed on the monitor has a certain correlation with the thickness of the thin film formed on each of the respective substrates on which a film to be formed 403. Ideally, the thickness of the thin film formed on the monitor is the same as the thickness of the thin film formed on each of the substrates on which a film to be formed 403, since the substrates on which a film to be formed 403 fixed in the domed substrate holder 402 are exposed incessantly to the film forming particles like the film formation monitor 8. Actually, the correlation is strictly obtained by an experiment. From the results, the correlation between the increment per unit time of the thickness of the thin film formed on the film formation monitor 8 and the increment per unit time of the thickness of the thin film formed on each of the substrates on which a film to be formed 403 is obtained.
Based on the measured thickness of the thin film, the film forming process control is carried out, such as adjusting the film forming rate during film formation, the refractive index of the films formed, etc., and closing the shutter to terminate film formation when a desired thickness has been reached.
In most film forming processes adopted especially for optical thin film application, substrates on which a film to be formed are held by a domed substrate holder rotating around a certain axis (may be virtual). Such a technique is described, for example, in Japanese Patent Laid-Open (Kokai) No. 1-306560.
Furthermore recently, also in a continuous production process in which a loading chamber and an unloading chamber are installed and connected with a film forming chamber to achieve higher productivity, a domed substrate holder is similarly used and rotated.
FIG. 6 is a typical view showing such a continuous production process. In this embodiment, a loading chamber 601, a film forming chamber 602 and an unloading chamber 603 are connected. Substrates on which a film to be formed are held by a domed substrate holder 604. One film forming unit consists of only the substrates on which a film to be formed held by the domed substrate holder 604. Each of the chambers can store one film forming unit. In this embodiment, during film formation, the next group of the substrates on which a film to be formed are supplied into the loading chamber, and the loading chamber is then evacuated. Concurrently, the unloading chamber is opened to the open air, and the substrates with a film formed are unloaded. Then, the unloading chamber is evacuated.
This technique is described, for example, in Japanese Patent Laid-Open (Kokai) No. 3-193873, etc. Furthermore a similar sputtering apparatus is described in Japanese Patent Laid-Open (Kokai) No. 7-278801, etc. However, in most of the continuous production apparatuses required to assure high accuracy in film thickness, etc., the substrate holder is fed to a film forming chamber (vacuum treatment chamber) and rotated there, and intermittent continuous operation is made with the time taken for film formation as the tact time. So the area available for film formation is used wastefully. This does not pose any large problem in the case of various optical lenses and small substrates. The reason is described below in reference to drawings.
FIG. 7 is a diagram showing the relation between the film forming area and the moving area of substrates on which a film to be formed. The film forming area 701 is usually almost circular. In the conventional method, a domed substrate holder as large as the circular film forming area 701 is prepared to hold substrates. That is, the film forming area in this case is inside the circular area 701. However, if the substrates on which a film to be formed are fed from left to right in the diagram, while having a film formed on each of them, the shaded portions of FIG. 7 can also be used as the film forming area. In the conventional method, only .pi./4 (78.5%) of the area available for film formation is used for production.
Furthermore, when the domed substrate holder is used, the problem becomes serious if the size of the substrates on which a film to be formed is larger. For example, in FIG. 8, an apparatus which has a circular film forming area with a diameter of 1 m is used to form a film on each of surface antireflection optical filters (26 cm.times.33 cm) for displays with a diagonal line length of 14 inches (35 cm). This state is seen from the position of the film formation monitor 8 in the direction toward the film forming particle flux axis 9. In this case, even if the conventional circular substrate holder is used most effectively, only five optical filters can be held. That is, in an area of 0.785 m.sup.2 available for film formation, only an area of 0.26.times.0.33.times.5=0.429 m.sup.2 is used for film formation. Only about 55% of the area available for film formation can be used for manufacturing such filters. If it is intended to raise the productivity of such filters, the manufacturing apparatus used must be larger than necessary.
Furthermore, there is a problem that if larger substrates are to be processed, the substrate loading efficiency declines. In addition, if the conventional method is used for forming a film on each of larger substrates, the apparatus used must be larger than necessary, and as a result, the evacuation system must also be larger as a further other problem.
Therefore, it can be said that production has been made using an apparatus remarkably low in productivity. The reasons why the method as shown in FIG. 4 has been used even though the productivity may decline remarkably like this are considered to be as follows.
The first reason can be that the properties of the films formed by the film forming particles flying in the direction of and around the film forming particle flux axis are more excellent than those of the films formed by the film forming particles flying in any other direction apart from said direction, and that the method has been believed to be most suitable for monitoring the film forming process. The reason for the excellent properties is that since the film forming particles flying in the direction of and around the film forming particle flux axis are large in number and generally have high kinetic energy, the films formed are high in refractive index and homogeneous. On the contrary, the film forming particles flying in any other direction away from the film forming particle flux axis are small in number and low in energy, and the films formed are small in refractive index and thin.
Therefore, it has been considered as a matter of course to place the film formation monitor near the film forming particle flux axis, and furthermore, in a design to rotate the substrates on which a film to be formed, it is most convenient to place the film formation monitor at the center of the substrate holder.
As the second reason, it has been believed to be a basic technique to rotate the substrate holder many times at an above position to form one-layer films to eliminate the unevenness otherwise caused by the conventional unstable film forming particle generation source. As a means for exposing the substrates on which a film to be formed to the film forming particles many times, the use of a rotary system has been mechanically simple.
A conventional apparatus which has overcome the low productivity of the intermittent continuous apparatus with a rotaty transfer type substrate holder is called a feed through type continuous apparatus or in-line apparatus. A feed through type film forming apparatus based on sputtering is proposed, for example, in Japanese Patent Laid-Open (Kokai) No. 5-106034, and a similar vacuum evaporator is proposed, for example, in Japanese Patent Laid-Open (Kokai) No. 6-65724. In these cases, the substrate holder (substrates) is once placed in a loading chamber called a load lock chamber, to stand by, and is fed into the film forming chamber intermittently continuously without releasing the vacuum of the film forming chamber. In these apparatuses, the time during which the substrates on which a film to be formed are exposed to vacuum as well as heat until a film is formed on each of them is very short (say, 10 minutes at the longest), and this poses a problem in the case of film formation on resin substrates, etc. In the case of resin substrates, it is often practiced to expose the substrates to heat or vacuum for a longer period of time, for degassing the substrates. If the degassing is not sufficient, such problems often occur that the adhesion between the resin substrate and the vapor-deposited film as adopted in the present invention is insufficient and that the gas evolved from the substrate during film forming process affects the film processing conditions, not allowing a desired film to be obtained. Furthermore, it can happen that when a transparent film for any of said applications is handled, visible light is absorbed, or that in the case of an optical film, the refractive index of the film is changed not to allow the desired spectral characteristics to be obtained. To solve these problems, in some cases, a vacuum chamber is arranged additionally on the loading side of the loading chamber, but it makes the apparatus very large (long in the direction in which the substrates on which a film to be formed are fed) disadvantageously in view of space.
When a multi-layer film such as a two-layer film is formed, for example as proposed in Japanese Patent Laid-Open (Kokai) No. 7-278801, a plurality of treatment chambers are provided, and a substrate holder is fed after lapse of a tact time to a treatment chamber, rotated there to have a film layer formed on it, and fed to the next treatment chamber after lapse of another tact time. In this case, vacuum treatment chambers as many as the kinds or number of film layers to be formed are used to make the apparatus large-sized disadvantageously. Furthermore, in the case of Japanese Patent Laid-Open (Kokai) No. 3-193873, etc., an idea of exchanging film forming particle generating materials is taken into account, but in this apparatus constitution, it is difficult to use large-sized substrates on which a film to be formed efficiently.
Moreover, as a feed through type continuous apparatus, it is proposed to arrange film forming particle sources in series, to obtain a multi-layer film by one time of feeding for film formation. However, in the case of a multi-layer film, especially in the case of an optical multi-layer film or a multi-layer film for an optical disc, etc., the film forming process conditions of one layer are often different from those of the previous layer or the subsequent layer, and it is very difficult to control the process conditions for obtaining a desired film. To solve this problem, for example, in Japanese Patent Laid-Open (Kokai) No. 62-260059, an orifice structure is used between the respectively adjacent vacuum treatment chambers, and it can be seen how difficult it is to separate the process conditions of different layers.
An apparatus in which the loading chamber contains in multi-stages substrates on which a film to be formed is proposed, for example, in Japanese Patent Laid-Open (Kokai) Nos. 64-4472 and 3-35216, etc., but since the film forming chamber does not have a structure to allow the substrates to be contained in multi-stages, that is, since the film forming chamber is separated from the loading chamber and the unloading chamber, continuous production cannot be effected. In the case of Japanese Patent Laid-Open (Kokai) No. 64-4472, since a mechanism to allow the target material to be exchanged is adopted for forming a multi-layer film, a multi-layer film can be formed, but since the loading chamber and the unloading chamber must be kept at atmospheric pressure to load and unload substrates on which a film to be formed and must be kept to stand by until they reach a vacuum condition equivalent to that of the film forming chamber, continuous production cannot be effected. In the case of Japanese Patent Laid-Open (Kokai) No. 3-35216, though the apparatus is intended to form a single-layer film, respectively two loading chambers and unloading chambers are used to allow continuous production. However, in this case, since the direction in which the substrates on which a film to be formed are loaded, the direction in which they are unloaded, and the direction in which they are fed for film formation are arranged three-dimensionally in three axes, a machine trouble is likely to occur when the substrates on which a film to be formed are changed in direction, and it is difficult to install the means for transferring the substrates in three directions in the apparatus. That is, it is very difficult to keep the transfer means sealed in vacuum, and the apparatus cannot be said to be suitable for practical production.